1. Field of the Invention
The present invention relates to a droplet ejection head driving method, a droplet ejection head and a droplet ejection device, and more particularly relates to an inkjet recording head and driving method for ejecting microscopic ink droplets with piezoelectric elements, and an inkjet recording device.
2. Description of the Related Art
A droplet ejection head which employs electromechanical conversion elements, such as piezoactuators (piezoelectric elements) or the like, can accurately control meniscus operations at a nozzle portion by applying a driving waveform to an electromechanical conversion element, and consequently has an advantage in being able to realize microdroplet ejections, control of satelliting/misting and the like.
In particular, a “pull-push” system, which draws a meniscus back into a nozzle immediately prior to droplet ejection and then performs ejection of the droplet, is extremely effective as a system for discharging microdroplets with very small droplet volumes (see, for example, the publications of Japanese Patent Nos. 3,275,965 and 3,159,188).
However, when a droplet ejection is performed by the above-described “pull-push” system, a phenomenon in which the meniscus greatly protrudes from a nozzle aperture just after the drop is ejected (a meniscus protrusion effect) occurs. This adversely affects frequency characteristics of droplet ejection, and there are problems in that ejections cannot be performed if a driving frequency is raised, and ejection stability characteristics, such as ejection direction, droplet size and the like, deteriorate.
Specifically, when the meniscus protrusion effect occurs just after droplet ejection, as shown in FIG. 3A, liquid protruding from an aperture portion of a nozzle 10 flows out onto a nozzle face, and enters a state in which the liquid wets surroundings of the nozzle 10 (a face flood state). When this face flooding occurs, there are problems in that it is not possible to perform ejections of droplets properly (and in worst cases there are ejection failures), and quality of a recorded image is greatly degraded.
Further, even if the face flood state shown in FIG. 3A is not reached, outflow of the liquid (wetting) may occur at a portion of the nozzle surroundings. In such a case, ejection of a droplet is possible but, as shown in FIG. 3B, a tail of a droplet 14 is drawn to one side, as a result of which a deterioration in an ejection direction characteristic occurs, which causes a reduction in quality of an output image.
In particular, if liquid-repellence of the surface around the nozzle 10 is low, that is, if a wetting characteristic is high, the problems described above are more likely to occur. Therefore, a liquid-repellent film with high quality and uniformity is required at the nozzle surface, and there is a resultant problem in that this leads to an increase in costs of the droplet ejection head.
Further, if a high liquid-repellence characteristic is maintained around the nozzle and overflowing of the liquid to the surroundings of the nozzle can be suppressed, a subsequent ejection still cannot be performed until a protruding meniscus 12, as shown in FIG. 3C, is returned to the nozzle aperture portion by the action of surface tension. Therefore, it is difficult to perform ejections of liquid droplets at high driving frequencies. As a result, a driving frequency of the head must be kept low, and processing capabilities of an overall device are reduced.
As described above, a conventional pull-push system has problems in being susceptible to the occurrence of the meniscus protrusion phenomenon just after droplet ejection, and consequently having difficulty with performing high-quality recording at high speeds.
A goal of the present invention is to solve the problems described above. Accordingly, for a droplet ejection head which performs droplet ejections by a pull-push system, a droplet ejection head driving method which suppresses meniscus protrusion just after droplet ejection and enables droplet ejection at high frequencies with excellent ejection stability characteristics is provided. An additional object of the present invention is to provide a droplet ejection device which can stably eject droplets with small droplet volumes at high frequency and can perform high-quality recording at high speed.
Conventionally, timings of voltage changes in a driving waveform have been implemented on the basis of an acoustic oscillation system, that is, of a resonance period (a Helmholtz oscillation period) Tc of a pressure wave which occurs in a pressure chamber. However, it has been established that there are two oscillation systems in an ejection head: the above-mentioned acoustic oscillation system and a refill oscillation system, which is oscillation of a meniscus due to surface tension at a nozzle.
The acoustic oscillations and the refill oscillations are both energized at the same time by application of a driving waveform. It has been learned that the problematic meniscus protrusion is caused by the latter, the refill oscillations, and a low-frequency meniscus oscillation caused by the refill oscillation system causes the meniscus to protrude just after droplet ejection.
Accordingly, the present invention will implement design of a driving waveform based on the refill oscillation system, that is, on a period Tm of meniscus oscillations that are caused by surface tension at a nozzle, and will effectively suppress meniscus protrusion.